While a tarp may seem like a convenient, readily available solution for protecting garden plants, its use requires careful consideration and is best reserved for very temporary, short-term needs. A traditional plastic or polyethylene tarp can function as a makeshift shelter to block wind or provide a slight thermal buffer against overnight frost. The primary function in these limited scenarios is to trap the heat radiating from the soil, preventing it from escaping into the cold night air and offering a few degrees of protection.
Immediate Risks of Improper Tarp Use
Using a non-breathable tarp, especially for extended periods or during the day, introduces risks that can quickly damage or kill plants. The most immediate danger is the creation of a “solar oven” effect due to the plastic’s inability to breathe. This causes temperatures beneath the cover to soar rapidly once the sun hits it, leading to overheating and tissue damage.
The trapped environment also causes excessive moisture to condense on the underside of the tarp. The resulting high humidity and constant moisture on the foliage create ideal conditions for the rapid proliferation of fungal diseases like mold and mildew. These pathogens thrive in stagnant, damp air, potentially causing more harm than the cold itself.
The physical weight of a standard tarp also threatens delicate plant structures. If the tarp rests directly on the foliage, the weight can crush stems and leaves, especially when the material becomes saturated with rain or accumulates heavy snow or ice. Direct contact can also cause the plant’s surface moisture to freeze more quickly, resulting in frost burn where the plastic touches the leaves.
Techniques for Safe, Temporary Coverage
To mitigate the risks of using a tarp, structural support is necessary to prevent direct contact with the plant. Building a simple frame using stakes, PVC pipe, or hoops ensures the tarp creates an insulating air pocket around the plant. This air space provides the thermal protection by holding in the earth’s residual warmth.
The tarp must be secured at the edges to prevent cold air from blowing underneath and displacing the warmer air pocket. It should not be sealed completely unless temperatures are extremely low, as minimal ventilation is advisable. The timing of removal is critical: the tarp must be lifted promptly at sunrise or as soon as the danger of frost has passed to prevent rapid internal temperature spike and moisture buildup.
When selecting a material, lighter colors or clear tarps are preferable for daytime use because they allow some light transmission and heat accumulation. Opaque, dark-colored tarps are better for blocking light entirely, which can be useful for extending the dormancy of certain plants. However, they absorb more solar radiation and lead to faster overheating, so for short-term frost defense, the focus remains on keeping the material off the foliage and ensuring timely removal.
Superior Alternatives for Plant Protection
In nearly all situations, specialized horticultural materials offer superior protection compared to a standard plastic tarp because they are engineered to be breathable. Floating row covers, often made from spun-bonded polypropylene or polyester, are a lightweight fabric that can be laid directly over plants without crushing them. Unlike a tarp, these covers allow light, air, and water to penetrate while offering 2 to 8 degrees Fahrenheit of frost protection by trapping heat.
These fabric alternatives create a protective microclimate that manages humidity rather than trapping it, significantly reducing the risk of fungal disease. The various weights of row covers, such as the common AG-19 material, transmit approximately 85% of sunlight. This means they can often be left on during the day without causing the heat stress seen under plastic. For a simple alternative, old cotton sheets or burlap can also be used, as their woven structure insulates while permitting the necessary air exchange.
For long-term, passive protection, robust structures like cold frames or cloches provide a mini-greenhouse environment without the risks of a draped plastic sheet. These enclosures use transparent materials over a fixed frame, allowing for better management of ventilation and temperature control. By choosing breathable fabrics or structured enclosures, gardeners can protect plants effectively while avoiding the moisture and overheating issues associated with non-porous tarps.